The present invention, in some embodiments thereof, relates to image processing. Specifically, the present embodiments can be used for providing an automatic dynamic range modulation of a digital image. In various exemplary embodiments of the invention the method and/or apparatus is used for companding (compressing and expanding) a high dynamic range (HDR) image. The present embodiments further comprise an imaging system.
High dynamic range imaging (HDRI) is a set of techniques that allow a far greater dynamic range of exposures (large difference between light and dark areas) than normal digital imaging techniques. The intention of HDRI is to accurately represent the wide range of intensity levels found in real scenes ranging from direct sunlight to the deepest shadows.
HDRI was originally developed for use with purely computer-generated images. Later, methods were developed to produce a HDR image from a set of photos taken with a range of exposures. With the rising popularity of digital cameras and easy to use desktop software, many amateur photographers have used HDRI methods to create photos of scenes with a high dynamic range.
HDR images require a higher number of bits per color channel than traditional images, both because of the linear encoding and because they need to represent values from 10−4 to 108 (the range of visible luminance values) or more. 16-bit (“half precision”) or 32-bit floating point numbers are often used to represent HDR pixels. However, when the appropriate transfer function is used, HDR pixels for some applications can be represented with as few as 10-12 bits for luminance and 8 bits for chrominance without introducing any visible quantization artifacts.
Digital images may contain a huge amount of data, especially for high quality display and printing. Commercially available digital imaging devices are known to acquire image information across a wide dynamic range of several orders of magnitude. Additionally, there are software solutions which fuse multiple exposures of the same scene at lower dynamic range into one image of higher dynamic range.
Typically, although at the time of image capture the acquired dynamic range is rather large, a substantial portion of it is lost once the image is digitized, printed or displayed. For example, most images are digitized to 8-bits (256 levels) per color-band, i.e., a dynamic range of about two orders of magnitude. The problem is aggravated once the image is transferred to a display or a print medium which is often limited to about 50 levels per color-band.
International Publication No. WO2009/081394, the contents of which are hereby incorporated by reference discloses an image processing technique in which a digital HDR image is processed using two adaptation procedures employed on the achromatic channel of the digital image. Each adaptation procedure incorporates a different effective saturation function of the intensity. The adaptation procedures mimic the operation of the physiological visual system, wherein the first procedure mimics the “on” retinal pathway and the second adaptation procedure mimics the “off” retinal pathways. The intensity level of each picture-element of the digital image is processed by both procedures. The result of each processing is an intermediate intensity level. All the intermediate intensity levels of the picture-element are then combined to provide a new achromatic intensity.